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Among other issues with Apple's "variable speed" approach is it requires the drive to spin up or slow down when it seeks to another track; this isn't a big deal when walking linerarly through the disk but it *is* a problem for random access. And again, it just doesn't result in an impressive capacity increase. Commodore used a similar GCR format to Apple for their drives, but with their system they varied the speed of the data, not the speed of the drive. (I have one of these drives, it's an impressive beast and the engineering of the controller board is fascinating.) And unlike Apple, their approach actually worked; the Commodore 8050 crammed almost 500k per side on a 77 track disk, which is essentially the maximum theoretical capacity of the disks.*

So, yeah. Apple's 40k more per disk was actually sort of pathetic considering how much customization they had to do to the stock Sony drive to accomodate it. I suspect the only reason they stuck with it was to salvage at least a little of the the investment they'd made in the infamous "Twiggy" drive's technology, and... maybe it worth it for them. If they'd simply formatted a fixed-speed drive with the same sector density as the Disk ][ drive (the IWM is essentially the same controller) the Sony would have only held about 310k per side (*less* than a PC floppy drive, it's worth noting); since the original Mac was explicitly designed *not* to support a hard drive because its floppies would have 871k available (the Twiggy's capacity) that much of a hit might have made abandoning the Twiggy unacceptable. In the end the 400k per side capacity they eeked out of their multiple-speed drives was little more than a face saving exercise.

(* Edit: One note about Commodore's approach: As impressive as it was said approach almost certainly wouldn't have sat well with Apple, despite the advantages, because it sort of requires a dedicated controller circuit instead of the completely software-driven approach Apple took with the Macintosh. On the Commodore drive there's actually a variable-speed clock circuit that speeds up or slows down depending on what track the disk is on, and said clock drives the dedicated CPU that talks to the disk drive. To make that work in a minimal hardware machine like the Mac you'd have to speed up or slow down the main CPU which would be... interesting, and probably not possible due to how the video display uses main memory.)

Re: Macintosh 400k - 800k Variable speed drives.

I agree the 10% size benefit probably wasn't worth the hassle of variable speeds for the original 400K/800K drives, but I'm not sure that explains why Apple dropped variable speeds from their later higher capacity formats. Apple's 1.44 MB superdrives had to retain all that variable speed BS in order to work with 400K and 800K media, so the engineering cost of a variable speed motor was already spent. It seems to me it was actually more work to do what they did: support 1.44MB single speed and 400K/800K variable speed disks in the same drive. They could have continued to use variable speed for all disk formats, and gotten 1.6 MB or so from their HD floppies, while saving some hassle by keeping the same method across the board for all floppy densities.

What would have made the most sense to me, actually, would be for Apple to have replaced their 400K/800K drives with standard PC 1.44 MB drives with no backwards compatibility for older floppies. Then tell people to use an external 400K/800K drive if they needed to read old floppies. That way they could use commodity PC floppy drives in their newer machines, and save on manufacturing costs.

Re: Macintosh 400k - 800k Variable speed drives.

I suspect they ditched variable speed on the Superdrive for two reasons:

A: Sort of the point was to be PC compatible, and:

B: The multi-speed drives spin between, what was it, 390 and 605 RPM?, while a normal HD floppy drive spins at 300RPM. The reason for the Mac floppy's high velocity was that Sony's microfloppy drives, as originally used on machines like the SMC-70 spun at 600RPM instead of the industry standard 300; this made data transfer quicker but also required clocking the floppy drive controller twice as fast as normal and was something of a barrier to their wide adoption. (Sony started producing 300RPM drives around 1984 or so, leading to their use in systems like the Data General One and other early PC laptops. Hewlett Packard's early Microfloppy disk drives like the 9121D, which actually beat the Mac to market by a fair stretch, used the 600 RPM versions.) What this means is that the Mac's disk system, even though it worked with "double-density" disks, was already running at high-density data rates. If Apple had wanted to use the same system on HD disks they'd either have to spin them half as fast (IE, *more* mechanical complexity) or double the data rate of the controller to almost 1Mbit/s. Considering they already had to redesign the controller so it could do MFM formatting they probably just decided it was time to cut their losses. (*Maybe* the IWM would be reliable if clocked twice as fast, and I suppose the could have used "extra high speed floppies!" as a selling point, but... eh.)

Re: Macintosh 400k - 800k Variable speed drives.

Eudimorphodon wrote:

(* Edit: One note about Commodore's approach: As impressive as it was said approach almost certainly wouldn't have sat well with Apple, despite the advantages, because it sort of requires a dedicated controller circuit instead of the completely software-driven approach Apple took with the Macintosh. On the Commodore drive there's actually a variable-speed clock circuit that speeds up or slows down depending on what track the disk is on, and said clock drives the dedicated CPU that talks to the disk drive. To make that work in a minimal hardware machine like the Mac you'd have to speed up or slow down the main CPU which would be... interesting, and probably not possible due to how the video display uses main memory.)

That may be true of the FDC in the older 2-CPU drives, but I don't think it's true of the 2031, 1541 and later drives with single "schizophrenic" CPUs. The 1541, at least, uses two clocks divided off a 16MHz crystal: a fixed 1MHz clock rate for the 6502, and a separate variable clock rate determined by track for the drive electronics (this is documented in the 1541 service manual). So Apple might have gotten away with something similar if they'd wanted to.

Re: Macintosh 400k - 800k Variable speed drives.

CHC: come to think about it, I think you're right; it's the "data pump" part of the circuit that has the variable clock connected to it. I guess I crossed that in my head with the other bizarre aspect of those old toaster-oven size drives, their use of two 6502-family CPUs, one dedicated to the data transfer mechanics and the other running DOS and driving the IEEE-488 port, and how they shared some blocks of memory without conflicts by being clocked out of phase with each other. (Yay for the 6502's magic 50% duty cycle synchronous bus.)

That being the case they probably could have worked something like that into the Mac's design and it might not have been the end of the world; as I recall, and again, recall clearly might be flawed, the Disk II controller was strictly synchronous and probably would have required some CPU clock-altering shenanigans but the IWM has at least some capability to act asynchronously. Peeking at the IWM spec document... the changes *may* have been limited to adding a variable clock circuit (presumably in place of the PWM signal circuit) and feeding the variable clock into the FCLK pin on the IWM?

(Gee, it can't have been that simple because otherwise it looks sort of stupid that they didn't do it when they adopted the SONY drive. Maybe they didn't want to have to redesign the logic board that late in the game?)